Chelates, Complexes & Salts of

Multivalent Cations in Aqueous Solutions

Presented by: Brian Haschemeyer

• Most micronutrients react with water with increasing alkalinity (pH)

• Calcium and Magnesium are stable in mild alkaline conditions

• Most all others react with water to form hydroxide salts.

Interactions with WaterHydrolysis of Multivalent Cations

0

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3 5 7 9 11

% F

ree

Met

al [

M]

pH of Aqueous Solution

pH Dependence of Metal Hydrolysis in Aqueous Solutions

Ca+2

Mg+2

Mn+2

Fe+2

Zn+2

Cu+2

Hydrolysis ReactionZn(OH) 2(s) Zn2+

(aq) + 2OH-(aq)

[M]+n Form Ksp mol %M

Ca+2 Ca(OH)2 5x10-6 1 4.1

Mg+2 Mg(OH)2 5x10-12 1 2.5

Mn+2 Mn(OH)2 2x10-13 1 5.5

Fe+2 Fe(OH)2 5x10-17 1 5.6

Zn+2 Zn(OH)2 3x10-17 1 6.6

Cu+2 Cu(OH)2 5x10-20 1 6.4

Nitrates & ChloridesHighly Soluble Sources

Zinc Chloride in Aqueous Solutions

pH

8642

% o

f Z

n+

+

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0

Zinc Nitrate in Aqueous Solution

pH

8642

% o

f Z

n+

+

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0

[Zn+2](aq)[Zn+2](aq)

Zn(OH)2(s)Zn(OH)2(s)

[Zn(NO3)]+

(aq)

[Zn(Cl)2]+

(aq)

[Zn(Cl)]+(aq)

Zn(Cl)2(s) Zn2+(aq) + 2Cl-

(aq)

Zn(NO3)2(s) Zn2+(aq) + 2NO3

-(aq)

[M]+n Form Equil const. mol %M

Zn+2 Zn(NO3)2 Log K -0.12 1 5.5

Zn+2 Zn(Cl)2 Log K 0.8 1 5.5

Phosphate InteractionspH Dependence of Phosphate binding

[M]+n Form Ksp

Ca+2 Ca3(PO4)2 1x10-26

Ca2(PO4)2(OH)2 1x10-27

Ca5(PO4)3(OH)3 1x10-57

Phosphate Interactions with Calcium vs pH

pH

6420

% o

f C

a+

+

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0

Acid Mol. Form pKa

H3PO4 H2PO4- 2.2

H1PO4-2 7.2

PO4-3 12.3

Dissociation of Phosphoric acid H3PO4

pH

14121086420

% o

f lig

an

d

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H3PO4 H2PO4- H1PO4

-2 PO4-3

Ca+2

Ca5(PO4)3(OH)3

Can I mix phosphate with CN9 (Calcium Nitrate) and other divalent cations??

Graph based on 200mmol phosphate & 20mmol Ca+2 concentrations

YES - If and only if the ph is dropped to 2.2 or below; depending on concentrations long term storage may not be good

How much do I need add to get to pH 2? 0.9% (w/w) solution of 54% Phos acid 1.3% (w/w) solution of PekAcid

There are stability constants for sulfate salts of divalent cations. Does this mean sulfate can complex??

Stability Constants (Log K Values)

Sulfate (SO4)-2

[ML]/[M][L]

H+ 1.98

Fe +2 2.20

Mg +2 2.25

Mn +2 2.30

Zn +2 2.28

SulfatesDo they complex????…..

Ferrous sulfate crystals Copper sulfate crystals

Manganese in Aqueous Solution

pH

86

% o

f M

n+

+

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0

Manganese Sulfate in Aqueous Solution

pH

86

% o

f M

n+

+

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0

Mn(OH)2(s) Mn(OH)2(s)[Mn(SO4)]+(aq)

Sulfate Coordination Suppressed AlkalineHydrolysis of Mn+2 in aqueous solutions

SulfatesDo they complex????….. To some extent yes…

[M]+n Form Equil const mol %M

Mn+2 MnSO4 Log K 2.2 1 5.5

Mn+2 Mn(OH)2 Ksp 2x10-13

[Mn+2](aq)[Mn+2](aq)

Effect of AMS on pH Dependence of Mn-Glyphosate Binding

pH

10864

% o

f M

n+

+

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pH Dependence of Mn-Glyphosate Binding

pH

10864

% o

f M

n+

+

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Glyphosate Micro CompatibilityGlyphosate vs Mn+2 with and without AMS

Thiosulfates are stable only in neutral or alkaline solutions, but not in acidic solutions, due to decomposition to sulfite and sulfur.

S2O32− (aq) + 2 H+ (aq) → SO2 (g) + S (s) + H2O

How low can you go with the pH and how long with it stay stable???

ThiosulfatesCalcium and Magnesium Solutions

A Liquid Sulfur product with Calcium and Magnesium, this is great.

So why not Thiosulfate formulations of Zinc, Manganese, Copper, etc. ??

Not for sure …. it may be possible if thiosulfate is stable in pH’s lower than 6.

Manganese Thiosulfate in Aqueous Solution

pH

864

% o

f M

n+

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[M]+n Form Equil const. mol %M

Mn+2 MnSO4 Log K 0.7 1 5.5

Mn+2 Mn(OH)2 Ksp 2x10-13

[Mn+2](aq)

Mn(OH)2(s)

[Mn(S2O3)]+(aq)

Ammonium

Equilibrium Reaction Log Kf

Cu2+ + 4NH3 [Cu(NH3)4]2+ 13.0

Zn2+ + 4NH3 [Zn(NH3)4]2+ 8.6

Ammonium (NH4+) is deprotinated to ammonia (NH3) in alkaline conditions so solutions need to be sufficiently alkaline (pH’s >9) to allow ammonia to complex metal. Common for use in polyphoshatesolutions. Ammonium is often used to complex metals in conjunction with other organic acids such as citric acid.

Stepwise Formation Constants

ion Kn log Kn

[Cu(NH3)(H2O)5]2+ K1 4.25

[Cu(NH3)2(H2O)4]2+ K2 3.61

[Cu(NH3)3(H2O)3]2+ K3 2.98

[Cu(NH3)4(H2O)2]2+ K4 2.24

[Cu(NH3)4(H2O)2]2+ Kf 13.08

Ammonia

pH

12108

% o

f lig

an

d

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NH4+

NH3

Citrate

Stability Constants (Log K Values)1

CITRATE

[MHL]/[M][H][L] [ML]/[M][L]

Al +3 11.8 8.1

Ca +2 7.6 3.4

Cu +2 9.5 6.7

Fe +2 8.7 4.5

Fe +3 12.4 11.2

Mg +2 7.2 3.2

Mn +2 7.1 3.7

Zn +2 8.7 5.0

1 R.M Smith; A.E. Martell, Crtical Stability Constants,

Plenum Press, New York and London, 3rd Edition.

Citric acid is natural biodegradable chelator. Because of its carboxyl groups, citric acid chelates / complexes metals in the acidic environment. Often used for formulation stability and foliar applications

Citrate

pKa Log Kf

pK3 6.1 6.1

pK2 4.6 10.7

pK1 3.1 13.8

Zinc Citrate 15mmol Zn : 20mmol Citrate

pH

1086420

% o

f Z

n+

+

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Free Zn++Zn(OH)2(s)

[M][H][L]

[M][L]

EDTA

pK1 pK2 pK3 pK4 pK5 pK6

0.0 1.5 2.0 2.66 6.16 10.24

Log Kf = 24.06 Log Kf = 22.56 Log Kf = 21.06 Log Kf = 19.06 Log Kf = 16.4 Log Kf = 10.24

Ion [M][H][L] [M][L]

Al +3 20.7 18.0

Ca +2 15.0 11.6

Cu +2 22.9 19.7

Fe +2 18.2 15.3

Fe +3 28.0 26.5

Mg +2 19.9 9.8

Mn +2 18.2 14.8

Zn +2 10.7 17.5

NH+

NH+

O OH

O

OH

O

OH

O OH

A Chelating agent that may have some advantage over EDTA for Iron in alkaline conditions and Magnesium in between pH 5 - 6

Stability Constants (Log K Values)1

HEDTA

[MHL]/[M][H][L] [ML]/[M][L]

Al +3 17.7 15.6

Ca +2 -- 9.0

Cu +2 20.8 18.3

Fe +2 15.8 13.0

Fe +3 -- 19.8

Mg +2 -- 7.0

Mn +2 10.9

Zn +2 -- 14.7

1 R.M Smith; A.E. Martell, Crtical Stability Constants, Plenum Press, New York and

London, 3rd Edition.

HEDTA

pKa Log Kf

pK3 10.3 10.3

pK2 5.6 15.9

pK1 2.8 18.7

HEDTAFor Magnesium in acidic conditions

Stability Constants (Log K Values)1

EDDHA

[MHL]/[M][H][L] [ML]/[M][L]

Al +3 - -

Ca +2 17.7 8.2

Cu +2 33.2 25.0

Fe +2 - 15.3

Fe +3 - 35.4

Mg +2 18.2 9.0

Mn +2 - -

Zn +2 25.8 17.81 R.M Smith; A.E. Martell, Critical Stability Constants, Plenum Press, New

York and London, 3rd Edition.

A high-performance chelating agent used in Agriculture particularly with iron in alkaline soil conditions.

EDDHA

pKa Log Kf

pK4 12.2 12.2

pK3 10.7 22.9

pK2 8.9 31.8

pK1 6.5 38.3

EDDHAFor Iron in alkaline conditions

GlucoheptonateIron and Copper in alkaline conditions

OH

OH

OH

OH

OH

OH

OH

O

O+ Na+

Glucoheptonates and Gluconates are biodegradable complexes/chelates that is useful

as an alternative for very high pH solutions, especially for Fe+3 and Cu+2

Stability Constants (Log K Values)1

[ML]/[M][L]

Glucoheptonate Gluconate

Al+3 -- --

Ca+2 1.25 2.2

Cu +2 41.2 38.9

Fe+2 1.1 1

Fe+3 38.3 37.2

Mg+2 0.78 0.7

Mn+2 -- --

Zn +2 1.82 1.7

Manganese Sulfate with glucoheptonate

pH

10864

% o

f M

n+

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Free Mn++

[M][[L]

[Mn(SO4)]+(aq)

Mn(OH)2(s)

Butch here has a strong complex on the football!!!! See, chelating is not that complicated. Even I can do it.

Zn+2

Mn+2

QUESTIONS??

Presented by: Brian Haschemeyer